The visual field is defined in general terms as the solid angle, with the apex at the aperture of the eye, through which vision is obtained. The visual field is approximately conical in a healthy individual, although slight variations in the shape of the visual field are common. It has been found that severe variations or disruptions in the shape of the visual field are often indicative of disease. Thus, the mapping of the visual field of an individual is an important tool in diagnosing such diseases as lesions and tumors of the brain, and defects in the retina and optic nerve.
The mapping of the visual field of the subject has always been approximate, due to the inaccuracies inherent in the methods of mapping. The most basic mapping technique is to provide a wall having a target screen, the target screen including a central fixation target and solid angle indicators. A moving spot of light is scanned across the target screen by the operator, who asks the individual when the spot disappears and when it can be seen. The operator must also ascertain that the subject is maintaining fixation on the central fixation target.
The opportunities for error in this method of visual field mapping are manifest, due primarily to the subjectiveness of the method of testing. It is difficult to ascertain that the subject is maintaining fixation on the central target, and it is also difficult to ascertain that the subject is truthfully responding to the test. Furthermore, the actual luminance of the moving spot, and the background illumination of the target screen, are rarely if ever measured. Thus the moving spot contrast level, which is quite important in determining the sensitivity as well as the area of the visual field, is unknown.
In more modern visual field mapping devices, a hemispheric screen is provided, with the head of the subject supported so that the eye is at the center of the hemisphere. A plurality of light emitting diodes may be selectively spaced about the hemisphere, and individually actuated to test the visual response of the eye. However, it is extremely difficult to vary the color of light emitting diodes, so that the visual field as a function of color cannot be mapped.
In other forms of visual field mapping devices known in the prior art, light conducting fibers extend from the back side of a hemispheric screen. The other ends of the light conducting fibers are joined to a board in a circular array, with the pivot of a light crank at the center thereof. The light crank is incremented to illuminate individual light conducting fibers, the crank being operated by a one-rotation clutch operated mechanism. Illumination is provided by an incandescent bulb which is driven by a variable transformer to vary the luminance selectively; filters and a shutter are also provided in the light path.
Although this prior art device appears to be quite sophisticated, it suffers from the same drawback as the crudest visual field mapping method. That is, there is no means for ascertaining the luminance level of the spots of light, and no control over the background illumination of the screen. The luminance level may be affected by the age of the incandescent bulb, dust in the optical system, and fluctuations of the line current supply. These variations significantly effect the outcome of the visual field mapping procedure, and are completely ignored.
Furthermore, there is no provision for ascertaining the exact position of the light crank with respect to the light fibers which it illuminates. As the light crank increments through one hundred positions to illuminate as many light fibers, cumulative errors in position may cause it to illuminate an incorrect light fiber, or to illuminate two light fibers at one time, or the like. These errors also are undetected.
Due to the inaccuracies of all of the prior art visual field mapping devices, it is impossible to replicate mapping tests with any degree of certainty. Mapping tests made on different devices or at different times may bear no relationship with other visual field maps for the same individual. Thus, most of the field mapping data which is accumulated is virtually worthless. Only the most gross visual field distortions can be considered significant. Subtle variations in visual field, or progressive variations indicative of progressive disease, cannot be detected.